class Base:
    def __init__(self):
        self.data = []
    def add(self, x):
        self.data.append(x)
    def addtwice(self, x):
        self.add(x)
        self.add(x)
# Child extends Base
class Child(Base):
    def plus(self,a,b):
        return a+b
#test    
oChild =Child()
oChild.add("str1")
print(oChild.data)
print(oChild.plus(2,3))
